Investing in altcoins have earned me most of my current money. For they grow the fastest and you can increase your capital several times. The main thing is not to risk much and invest in reliable altcoins. For example, in the Altcoins from the top 50 list.
We all buy altcoins to make money, but I dont think it would be right to call this a useful application for its intended purpose. Any project becomes useful when it implements its plans and becomes a working business, there are a lot of such companies already operating in crypto market to say that not all altcoins are useless